This invention relates to phased array antennas, and more particularly to distributed arrangements for control of the beam direction.
Directional antennas are widely used in a variety of detection and communication applications. High gain can be achieved in an antenna system by the use of a relatively simple feed antenna directed toward a shaped reflector. Those skilled in the antenna art know that the effective aperture of the reflector and its shape determine the beam shape and gain. Such antennas are passive, in that they contain no unidirectional elements, and therefore their performance in a transmission and reception modes are identical, even though their descriptions may be couched in terms of either transmission or reception.
When antenna gain must be very high, the aperture subtended by the reflector must be large, and as known such reflectors have a three-dimensional curvature. The large size and curved nature of the reflectors makes them heavy and prone to damage due to wind loading. Also, the inertia of the mass of the reflector demands large motors and substantial power when rapid slewing of the antenna is required to redirect the beam.
Phased-array antennas solve some of the problems associated with reflector-type antennas. For a given gain, the aperture of the phased array antenna is required to be about the same as the aperture of the reflector antenna. However, the phased-array antenna can be made as an essentially flat structure, which lends itself to applications on moving vehicles, and makes it less susceptible to lateral wind movements in ground-based applications. The phased-array antenna includes a plurality of elemental antennas arranged in an array, and fed with a particular phase and amplitude distribution as required to achieve the desired performance. The beam direction of a phased-array antenna may be selected by selection of phase shifts, which can be performed electronically. Thus, the antenna structure can be fixed.
As described above, the phased-array antenna may be reciprocal. When high transmitted power is desired, each antenna element of the phased array may be associated with an independent amplifier, to thereby form an "active" array. In such an active array, the total transmitted power represents the cumulated power of the amplifiers. In a receive mode, the losses in the phased-array feed system, which would otherwise result in degraded noise performance of the antenna, may be overcome by the use of a low-noise amplifier associated with each elemental antenna, for preamplifying the signal received at each antenna before it is attenuated by the feed system losses. The transmit and receive amplifiers are unidirectional, so that the parameters of the phased array antenna as a whole may be different in the transmission and reception modes. The transmit and receive amplifiers, and the controllable phase shifters, may be located in a transmit-receive (TR) module associated with each elemental antenna. U.S. Pat. No. 5,103,233, issued Apr. 7, 1992, in the name of Gallagher et al, describes a radar system which takes advantage of some of the properties of active phased-array antennas to achieve high-speed volume surveillance in an air traffic control radar context.
As described in the above mentioned Gallagher et al patent, the phase shifters and attenuators of each transmit-receive module associated with each elemental antenna may be controlled from a central location. As also mentioned therein, the central location tends to require a substantial data flow between the central control unit and each transmit-receive module. A distributed scheme is also described in the Gallagher et al patent, in which each elemental antenna is associated with a controller which performs many of the computations which would otherwise have to be performed at the central computer. This arrangement allows the central computer to transmit simpler commands to the various phase shifters than in the centralized system, thereby reducing the amount of data flow through the system by transferring substantial amount of the computations to the individual TR modules.
An improved phased array antenna system is desired.